To ensure the continuity of a voice call between a circuit switched (CS) domain and an internet protocol (IP) multimedia core network subsystem (IMS) domain, i.e. the voice call continuity (VCC), a traditional solution is as shown in FIG. 1:
A dual mode user equipment (UE) is registered both in the CS domain and the IMS domain. When the UE initiates a voice call in the CS domain or the IMS domain, the voice call is equally anchored at a voice call continuity application server (VCC AS) in the IMS domain. The dual mode UE keeps two numbers which are known to the VCC AS: a VCC Domain Transfer URI (VDI), indicating that a user is transferred from the CS domain to the IMS domain; and a VCC Domain Transfer Number (VDN), indicating that a user session is transferred from the IMS domain to the CS domain at a service layer. When the dual mode UE has initiated a voice call in the CS domain, and prepares to be transferred from the CS domain to the IMS domain, the dual mode UE uses the VDI as the called number in the IMS domain, and initiates a session request to the VCC AS; after receiving the session request, the VCC AS releases the resources in the CS domain, and associates the resources between established session and original session in the IMS domain, thus the transferring is completed. Likewise, when the dual mode UE has initiated a voice call in the IMS domain, and prepares to be transferred from the IMS domain to the CS domain, the dual mode UE uses the VDN as the called number in the CS domain, and initiates a session request to the VCC AS; after receiving the session request, the VCC AS releases the resources in the IMS domain, and associates the resources in IMS between established session and original session in the IMS domain, thus the transferring is completed.
The above-mentioned contents describe the traditional principles for realizing VCC, in which the dual mode UE must support the Dual Radio, i.e. voice calls can be realized simultaneously both in IMS domain and CS domain.
To realize IP Multimedia Subsystem (IMS) centralized services (ICS) in the IMS domain, an application server IMS CS Control Function (ICCF) is added in the IMS domain, the ICCF is adjacent to the VCC AS, and is located at the first application server (AS) for initiating a call, while located at the last AS for terminating a call. Other CS network is as a channel to access IMS domain, and all services are steered and triggered in the ICCF of the IMS domain.
With the flat development of the network architecture, and appearances of multiple mobile access modes of high-speed bandwidth, the next generation architecture concept of the LTE/SAE is impelled to be proposed for the mobile communication network, so that an evolved wireless communication system can provide higher transmission rate, shorter transmission delay and lower cost, and meanwhile support the mobility among internal access systems of 3rd generation partnership project (3GPP), and the mobility between a 3GPP access system and a non-3GPP access system. In addition, the LTE/SAE is also used as an access means of the IMS domain, and supports the original VCC functions. However, due to reasons such as great energy consumption of mobile phone batteries and powerful mobility management capability of the LTE/SAE itself under dual modes, the single radio voice call community (SR VCC) is proposed, and currently the principle architectures of the two schemes for realizing SR VCC are as shown in FIG. 2 and FIG. 3, wherein:
the corresponding network elements of the LTE/SAE mainly include:
an evolved UTRAN (E-UTRAN) is a next generation wireless access network, which can provide higher uplink and downlink rates, lower transmission delay and more reliable wireless transmission.
A mobility management entity (MME) is a functional entity for a control plane, and is responsible for the management and storage of the UE contexts (e.g. a UE/user ID, a mobility management state, user security parameters, etc.); the MME assigns temporary identifications for the users, and is responsible for authenticating the users when the UE resides in the tracking area or in the network.
An SAE gateway (SAE GW) is a functional entity for a user plane, and responsible for the routing processing of the user plane data, and can be classified into a Serving SAE GW (SGW) and a Public Data Network Gateway (PDN GW). The SGW is a mobile anchor point between an SAE system and a traditional 3GPP system, and is also a mobile anchor point between the SAE system and the E-UTRAN. The PDN GW terminates the UE downlink data which is in an idle state, and initiates a paging when the downlink data sent to the UE arrives; the PDN GW manages and stores the UE context, such as IP bearer service parameters and routing information within the network etc.
The existing 2nd Generation (2G)/3rd Generation (3G) mobile network mainly consists of the following network elements:
a 2G GSM/EDGE radio access network (GERAN) or 3G universal terrestrial radio access network (UTRAN), and a core network (CN). Network elements of the core network contain a CS domain network element that is a visited mobile services switch center (VMSC)/mobile services switch center (MSC) and a packet switched (PS) domain network element that is a serving GPRS support node (SGSN), wherein the VMSC/MSC provides circuit voice services, circuit data services and short message services; the SGSN is connected with the SAE GW of the LTE/SAE, and provides services in the PS domain. A home subscriber server (HSS) manages data of the CS domain, the PS domain and the IMS domain.
An IMS network mainly includes the following network elements:
core network elements such as a call session control function (CSCF), a media gateway control function (MGCF) etc., and a VCC AS for realizing the VCC functions.
The above-mentioned contents describe the networks of the existing technologies. To realize single radio voice call continuity, a single-radio-voice-call-continuity control function is added, such as a shared inter working function (S-IWF) as shown in FIG. 2, and an inter working function (IWF) as shown in FIG. 3; the S-IWF/IWF is connected with the network element VMSC in the CS domain and network element SGSN in the PS domain of the 2G/3G core network, and the MME in an LTE/SAE network; and the S-IWF/IWF controls the switching between the LTE/SAE network and 2G/3G network. The S-IWF/IWF is connected with the ICCF/VCC AS via the CSCF in the IMS domain network, to make sure that all calls can be anchored at the ICCF/VCC AS in the IMS domain, and guarantee the realization of VCC basic functions.
Wherein, in the scheme as shown in FIG. 2, the interface between the S-IWF and the VMSC is an E-interface, and mobile application part (MAP) signals are adopted in the E-interface, an inter-office handover mode of the CS domain is adopted in the handover between the S-IWF and the VMSC; in the scheme as shown in FIG. 3, the interface between the IWF and the VMSC is an A/Iu-CS Interface, and an intra-office handover mode in the CS domain is adopted in the handover between the IWF and the VMSC.
In the above-mentioned network architectures, the UE can access the IMS domain via the LTE/SAE, and also can access the IMS network via the PS domain (i.e. the access equipment SGSN of the PS domain as shown in the figure) of the 2G/3G network (e.g. the access equipment GERAN/UTRAN of the CS domain as shown in the figure), to enjoy IMS services, and meanwhile the UE can access the 2G/3G CS network (i.e. the VMSC/MGW as shown in the figure), and provides CS services such as voice calls etc.
FIG. 4 is a flowchart illustrating the process for realizing the transferring from an LTE/SAE to a CS domain under an SR VCC architecture as shown in FIG. 2, and a session is initiated between a UE and a UE-B in an IMS, and is anchored at a VCC AS; the transferring process as shown in FIG. 4 includes the following steps:
101, a UE sends a measurement report to a radio network element E-UTRAN of an LTE/SAE;
102, the E-UTRAN initiates a relocation required message to an MME according to the information included in the measurement report;
103, the MME sends a forward relocation required message to an S-IWF according to the information included in the relocation required message, and indicates the S-IWF to handover the corresponding session to a CS domain and make the session access from the CS domain;
104, the processes of requesting a handover and requiring a handover and responses thereof for an inter-office handover of the CS domain among the S-IWF and CS domain network elements of a VMSC and a GERAN/UTRAN, are completed;
the CS domain inter-office handover includes: the S-IWF sends a MAP-PREPARE-HANDOVER request message to the VMSC, and the VMSC sends a handover request message to the GERAN/UTRAN, then the GERAN/UTRAN sends a handover request acknowledge message back to the VMSC, and the VMSC sends a MAP-PREPARE-HANDOVER acknowledge message back to the S-IWF, and the message includes a handover number (HO number);
105, the S-IWF sends an initial address message of ISDN user part (ISUP) signals to the VMSC according to the handover number, and the VMSC sends an address complete message of the ISUP back to the S-IWF;
106, the S-IWF sends the initial address message of the ISUP to an ICCF/VCC AS via the VMSC which is in the CS domain and an MGCF and a CSCF which are in an IMS domain. The message includes a VDN, which indicates that the session is transferred from the IMS domain to the CS domain in a service layer, and the ICCF/VCC AS sends an answer message back to the S-IWF;
107, the ICCF/VCC AS sends a session release message to the UE via the IMS domain and an LTE/SAE network, and notifies the UE-B to update the bearer via the IMS domain;
108, the S-IWF sends a forward relocation request acknowledge message back to the MME;
109, the MME sends a relocation request acknowledge message back to the E-UTRAN;
110, the E-UTRAN sends a relocation command message (handover command) back to the UE;
111, the UE sends a handover access message to a target GERAN/UTRAN;
112, after detecting the handover, the GERAN/UTRAN sends a handover detected message to the VMSC;
113, the VMSC sends a MAP-PROCESS-ACCESS-SIGNALLING request message to the S-IWF, the message is used to notify the S-IWF that the UE has started to access from the CS domain;
114, the UE sends a handover completed message to the GERAN/UTRAN;
115, the GERAN/UTRAN sends a handover completed message to the S-IWF via the VMSC;
116, the VMSC sends a MAP-SEND-END-SIGNAL request message to the S-IWF, to notify the S-IWF that the UE has been transferred to the CS domain, and the S-IWF determines the completion of the transfer from the MAP-SEND-END-SIGNAL request message;
117, the VMSC sends an answer message of the ISUP (corresponding to the initial address message of the ISUP in step 105) back to the S-IWF;
118, after the handover is completed, the S-IWF notifies the LTE/SAE network to release resources.
If the VCC AS has released LTE/SAE resources in step 107, step 118 is no more executed.
FIG. 5 is a flowchart illustrating the process for realizing a call transfer from an LTE/SAE to a CS domain under SR VCC architecture as shown in FIG. 3, mainly including the following steps:
S101, a session is initiated between a UE and a UE-B in an IMS, and is anchored at a VCC AS;
S102, the UE attaches to a CS network via the LTE/SAE network;
S103, the UE initiates a CS call via the LTE/SAE network, and the access process (including encryption and authorization) is completed;
S104, the UE sends a call setup message (e.g. SETUP message) to a network side (e.g. IWF), the message includes a VDN as a called number;
S105, the IWF saves the call setup information;
S106, the UE sends a measurement report to an LTE/SAE radio network element E-UTRAN;
S107, the E-UTRAN decides to perform a handover;
S108, the E-UTRAN sends a handover request message to an MME according to the information included in the measurement report;
S109, the MME sends a handover request message to the IWF;
S110, the IWF sends a call setup message (e.g. a SETUP message) to a VSMC according to the information saved in step S105, and the message includes a VDN as a called number;
after step S110, steps S111˜S115 and steps S116˜S128 will be respectively executed simultaneously:
S111, according to the VDN in the call setup message, a VMSC sends an initial address message of the ISUP to set up a call to an MGCF;
S112, after receiving the ISUP initial address message, the MGCF sets up a SIP (session initial protocol) session to the VCC AS;
S113, the MGCF sends an ISUP answer message to the VMSC after the SIP session is set up successfully;
S114, the VMSC sends an answer message (or CONNECT message) to the UE;
S115, the VCC AS initiates a release session message to the UE via the IMS domain and the LTE/SAE network, and notifies the UE-B to update the bearer via the IMS domain;
S116, the VMSC sends a channel assignment request message to the IWF, and the IWF sends a channel assignment response message back to the VMSC, thus the channel assignment process is completed;
S117, then the IWF sends a handover request message to the VMSC according to the information received in step S109;
S118, the VMSC sends a handover request message to a target GERAN/UTRAN;
S119, the target GERAN/UTRAN completes the resources reservation, and sends a handover response message to the VMSC;
S120, the VMSC sends a handover command message to the IWF;
S121, the IWF sends a handover response message to the MME;
S122, the MME sends a handover command message to the E-UTRAN;
S123, the E-UTRAN sends a handover command message to the UE;
S124, the UE sends a handover access message to the target GERAN/UTRAN, to access the target GERAN/UTRAN;
S125, after detecting the handover, the GERAN/UTRAN sends a handover detected message to the VMSC;
S126˜S127, the handover of the UE is completed; the VMSC receives a handover complete message;
S128, the VMSC releases resources of the handover initiating side LTE/SAE.
The following problems exist in the above-mentioned technologies: after receiving the initial address message of the ISUP (step 106 and step S111), the ICCF/VCC AS will release the session set up between the ICCF/VCC AS and the UE via the LTE/SAE, while here the UE may have not received the handover command, or may be in the process of handover, and the session path is as shown in FIG. 6, only the link from the CS network to the remote node is connected between the UE and the remote node. The session between the UE and the remote node can be set up only after the UE is transferred to the CS domain successfully; if the UE is not transferred to the CS domain successfully, then the UE is unable to return to the original session path to continue the session as the session resources between the VCC AS and the LTE/SAE network have been released.